tidying up rule data structures

haskell-experiments/src/Datalog/NaiveDatabase.hs

@@ -11,12 +11,10 @@ import Data.Map (Map)
 import Data.Set (Set)
 import Data.Set qualified as Set
 import Data.Text (Text)
-import qualified Data.Text as T
 import Datalog.DatalogParser(parseDatalog, Literal(..), Statement(..), Term (..), Head (HeadSingle))
 import qualified Data.Map as Map
 import Text.Megaparsec (ParseErrorBundle)
 import Data.Void
-import Control.Exception (Exception)
 import Control.Exception.Base
 import Data.List
 import Data.Maybe
@@ -36,7 +34,7 @@ data RuleElement = -- entry occurring in a head or body relation - constant or v
   deriving (Show, Eq)
 
 data RelationRule = RelationRule {
-  headVariables :: [RuleElement],
+  headVariables :: [Text],
   body :: [(Relation, [RuleElement])]
 } deriving (Show, Eq)
 
@@ -195,7 +193,9 @@ withFactsAndRules facts rules =
         toConstraintElement :: RuleElement -> ConstraintElement
         toConstraintElement (RuleElementConstant constant) = ConstraintElementConstant constant
         toConstraintElement (RuleElementVariable varName) = ConstraintElementIndex (varIndex varName)
-
+        extractVarName :: RuleElement -> Text
+        extractVarName (RuleElementVariable varName) = varName
+        extractVarName (RuleElementConstant constant) = throw $ UnexpectedConstantException constant
         -- toBodyConstraint :: (Relation, [RuleElement]) -> BodyConstraint
         -- toBodyConstraint (subRelation, ruleElements) =
         --   BodyConstraint {
@@ -203,7 +203,7 @@ withFactsAndRules facts rules =
         --     _elements = toConstraintElement <$> ruleElements
         --   }
         newRule = RelationRule {
-          headVariables = variables',
+          headVariables = extractVarName <$> variables',
           body = fromBodyConstraint <$> (_bodyConstraints context'')
         } where
           fromBodyConstraint :: BodyConstraint -> (Relation, [RuleElement])
@@ -237,7 +237,8 @@ data NaiveDatabaseException
     NonRuleException Text Statement |
     NonQueryException Text Statement |
     BadArityException Text Int |
-    VariableLookupException Text [RuleElement]
+    VariableLookupException Text [RuleElement] |
+    UnexpectedConstantException Constant
   deriving (Show)
 
 instance Exception NaiveDatabaseException

haskell-experiments/test/Test/Datalog/NaiveDatabaseSpec.hs

@@ -53,11 +53,7 @@ spec = do
               ],
               _rules = [
                 RelationRule {
-                  headVariables = [
-                    RuleElementVariable "X",
-                    RuleElementVariable "Y",
-                    RuleElementVariable "Z"
-                  ], 
+                  headVariables = [ "X", "Y", "Z" ], 
                   body = [
                     (
                       Relation {